In my last two posts, I've been careful to use the word 'transportation' instead of 'travel,' and with good reason: My goal is to highlight the gameplay differences between the two.
Transportation lets players move among places they've already been. In some cases, transportation lets players move among places that their characters would have had easy access to - even if the player is new to the place.
Players travel when they go somewhere new. When players successfully travel through a new area, they often earn new modes of transportation.
However, the line gets blurred fairly easily. For example, when players choose to take 'the long way' or 'fight their way' through an area they've already been, I would say that the players are traveling.
Another blurry line appears when players use their own mounts or vehicles. If players can whiz by content that would put them in danger if they weren't on their mount/vehicle, then I'd say that's transportation. However, if the use of the mount/vehicle only causes players to encounter danger more frequently, then I'd call that travel.
Links to my other posts on the subject: